The vast majority of vehicles currently in use incorporate vehicle communication systems for receiving or transmitting signals. For example, vehicle audio systems provide information and entertainment to many motorists daily. These audio systems typically include an AM/FM radio receiver that receives radio frequency (RF) signals. These RF signals are then processed and rendered as audio output. A vehicle communication system may incorporate other functions, including, but not limited to, wireless voice and data communications, global positioning system (GPS) functionality, satellite-based digital audio radio service (SDARS) functionality, keyless entry, and remote vehicle starting.
Communication systems, including vehicle communication systems, typically employ antenna systems including one or more antennas to receive or transmit electromagnetic radiated signals. In general, such antenna systems have predetermined patterns and frequency characteristics. These predetermined characteristics are selected in view of various factors, including, for example, the ideal antenna design, physical antenna structure limitations, and mobile environment requirements.
Some antenna modules incorporate multiple antennas for use in different applications, including cellular telephony in the AMPS, PCS, and GSM communication bands, digital audio broadcast (DAB), GPS, and SDARS systems. These antennas can be stacked or placed on the same circuit board, for example, arranged adjacent to one another in a row.
One type of antenna, known as an antenna mast, is commonly used in high frequency communications. For example, antenna masts may be used in wireless voice and data communications systems operating at frequencies up to and even in excess of 1 GHz. An antenna mast may be implemented, for example, as a flexible fiberglass or TEFLON® rod with a helically-wound conductor for receiving radio signals.
Small mast antennas, such as those used for cellular telephony, distort the radiation pattern for satellite services, e.g., GPS, and SDARS, due to coupling and shadowing. However, due to the small size of DAB and cellular telephone antenna masts, this distortion is normally acceptable. On the other hand, larger mast antennas, such as those used for AM and FM radio reception, present significantly more distortion in the radiation patterns for satellite services. Accordingly, larger mast antennas cannot be placed adjacent to satellite antennas.
To address this issue, some conventional solutions incorporate a concentric antenna arrangement. In such an arrangement, a helical antenna for use with satellite services such as GPS and SDARS is arranged concentrically with a mast antenna for reception of terrestrial signals, such as DAB or cellular telephone signals. The helical antenna, which operates at its axial mode, can be constructed using a thin, flexible substrate or a wire wrapped into a cylindrical shape. The cables or wires for the mast antenna are typically routed inside and concentric to the helical antenna. Accordingly, the mast antenna is thicker and more rigid than traditional AM/FM mast antennas. One drawback to this type of design is that the helical antenna cannot be bent along its length. In addition, if the mast antenna needs to be tilted for a particular vehicle application, the helical antenna must also be tilted, potentially resulting in an unacceptable radiation pattern for the helical antenna.